The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods and apparatus. Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 typically has allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity. Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
In a typical DS-CDMA system, data streams from different users are subjected to various signal processing steps, such as error correction coding or interleaving, and spread using a combination of a user specific spreading code and a group-specific scrambling code. The coded data streams from the users are then combined, subjected to carrier modulation and transmitted as a composite signal in a communications medium.
A RAKE receiver structure is commonly used to recover information corresponding to one of the user data streams. In a typical RAKE receiver, a received composite signal is typically correlated with a particular spreading sequence assigned to the receiver at each of a plurality of correlation times (e.g., delays) to produce a plurality of time-offset correlations, a respective one of which corresponds to an echo of a transmitted spread spectrum signal. The correlations are then combined in a weighted fashion, i.e., respective correlations are multiplied by respective weighting factors and then summed to produce a decision statistic.
The performance of CDMA systems generally is limited by interference among different user signals. Spreading/despreading provides a degree of interference suppression, but the number of users is generally limited by interference. Conventional RAKE reception techniques generally treat interference as white noise. More recently proposed techniques provide for a degree of interference cancellation through xe2x80x9cwhiteningxe2x80x9d of interference. Examples of such techniques are described in xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part I: Theory and Background,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 12, pp., 817-827(June 1994); xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part II: Implementation Issues,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 14, pp. 1488-1499 (October 1996); xe2x80x9cData Detection Algorithms Specifically Designed for the Downlink of CDMA Mobile Radio Systems,xe2x80x9d by Klein, 1997 IEEE Vehicular Technology Conference, Phoenix Ariz. (May 4-7, 1997); U.S. Pat. No. 5,572,552 to Dent et al. (issued Nov. 5, 1996); and xe2x80x9cOptimizing the Rake Receiver for Demodulation of Downlink CDMA Signals,xe2x80x9d by Bottomley, Proceedings of the 43rd IEEE Vehicular Technology Conference, Secaucus N.J. (May 18-20,1993).
According to embodiments of the present invention, correlation times are determined from time differentials between times associated with multipath components of a signal based on correlation metrics, preferably signal strength measurements, associated with the multipath components. According to various embodiments of the present invention, selection strategies are employed in which xe2x80x9cdesired signal collectingxe2x80x9d and xe2x80x9cinterference collectingxe2x80x9d correlation times may be selected using average optimal (AO) or instantaneous optimal (IO) selection criteria. These criteria may include, for example, thresholds for signal strengths associated with multipath components of a signal at the correlation times, where the signal strengths may include absolute or relative measures of signal power or signal to noise ratio. According to alternative embodiments, correlation times are selected using an inverse filter of an estimated channel response.
Copending U.S. patent application Ser. Nos. 09/344,898 and 09/344,899, each of which was filed Jun. 25, 1999, describe RAKE-type receivers that utilize interference rejection combining (IRC) or interference whitening (IW) techniques that offer improved performance in the presence of interference. The present invention arises from the realization that performance of these and other RAKE receivers can be improved by judicious selection of the correlation times (delays) that are used in the receiver.
In particular, according to one embodiment of the present invention, respective correlation metrics are generated for respective ones of a plurality of multipath components of a first signal. Respective time-offset correlations of a second signal with a modulation sequence are generated at respective correlation times that are determined from time differentials between times associated with the plurality of multipath components based on the correlation metrics associated with the plurality of multipath components. In preferred embodiments according to the present invention, the first signal is processed to determine respective signal strengths of respective ones of the plurality of multipath components of the first signal. Respective time-offset correlations of the second signal with the modulation sequence are generated at correlation times determined from time differentials between times associated with the plurality of multipath components based on the determined signal strengths of the plurality of multipath components.
According to embodiments of the present invention, a two stage correlation time selection approach is used in which xe2x80x9cdesired signal collectingxe2x80x9d correlation times are used to determine additional xe2x80x9cinterference collectingxe2x80x9d correlation times that can aid in interference canceling or whitening. A first signal is correlated with a modulation sequence at respective ones of a set of first, xe2x80x9cdesired signal collecting,xe2x80x9d correlation times to generate respective first correlation outputs. Respective signal strengths are determined for the first correlation outputs. A second, xe2x80x9cinterference collecting,xe2x80x9d correlation time is determined based on the determined signal strengths for the first correlation outputs. Respective time-offset correlations of the second signal with the modulation sequence are generated at the first and second correlation times.
According to another aspect of the present invention, selection strategies are used in which the second correlation time is determined from time differentials between the first correlation times based on determined signal strength criteria. One first correlation time of the set of first correlation times that has a signal strength meeting a predetermined criterion is identified. At least one time differential between the identified one first correlation time and at least one other first correlation time of the set of first correlation times is then identified. The second correlation time is determined from the at least one time differential.
In another embodiment of the present invention, respective correlation metrics are determined by generating an estimated channel response from a first signal. An inverse filter of the estimated channel response is then determined, the estimated channel response including respective ones of a plurality of inverse filter coefficients and respective delays associated therewith. Respective time-offset correlation of a second signal with the modulation sequence are generated at respective correlation times selected from the plurality of delays based on the inverse filter coefficients. The estimated channel response may include an instantaneous channel response estimate or an average channel response estimate.
According to yet another aspect of the present invention, a receiver includes a correlation timing determiner, responsive to a received signal, that generates respective correlation metrics for respective ones of a plurality of multipath components of a first signal, respective ones of the plurality of multipath components having respective correlation times associated therewith, and determines a set of correlation times from time differentials between the times associated with the plurality of multipath components based on the correlation metrics. A correlation unit, operatively associated with the correlation timing determiner, generates respective time-offset correlations of a second signal with a modulation sequence at respective correlation times of the selected set of correlation times. A combiner combines the time-offset correlations of the second signal with the modulation sequence to produce an estimate of information represented by the second signal. In one embodiment of the present invention, the correlation timing determiner includes a search correlation unit that generates respective correlation outputs for respective ones of the multipath components. A signal strength determiner determines respective signal strengths of respective ones of the plurality of multipath components of the first signal. A correlation time selector selects the set of correlation times for use in the correlation unit based on the determined signal strengths.